Bone fracture support implant with non-metal spacers

ABSTRACT

An intramedullary nail structure is formed with opposing dynamization windows, and spacers of a bioresorbable material are positioned within the dynamization windows. The dynamization windows are longer than they are wide. The spacers may be integrally formed as a single insert. The nail is used with a bone fastener such as a bone screw which is advanced transversely through the bone and into the spacer, preferably in a bicortical attachment with the bone. The bone fastener is smaller across than the dynamization windows, so each spacer spaces the bone fastener relative to its dynamization window. As the spacers resorb, stress (at least in one direction) is increasingly transmitted through the fracture site rather than through the intramedullary nail. The positioning of the bone fastener, the shape and size of the dynamization windows and spacers, and the material of the spacers all allow design control over the type and amount of dynamization seen at the fracture site. Also, because the bone fastener is smaller across than the dynamization windows and spacers, a larger error in placement of the bone fastener is permissible.

The present application is a continuation of application Ser. No.09/289,324, filed Apr. 9, 1999 and entitled INTRAMEDULLARY NAIL WITHNON-METAL SPACERS, now issued as U.S. Pat. No. 6,296,645.

CROSS-REFERENCE TO RELATED APPLICATION(S)

None.

BACKGROUND OF THE INVENTION

The present invention relates to intramedullary nails used for treatmentof a fracture of a bone having a medullary canal extendinglongitudinally within the bone, and particularly to the structure of theintramedullary nail and methods for anchoring the intramedullary nailwith respect to one or more fragments of the fractured bone.

Intramedullary nails are used by orthopedic surgeons to treat fracturesinvolving long bones such as the femur, humerus, tibia, fibula, etc. Themedullary canal of the fractured bone is drilled out or otherwise openedfrom one end, and the intramedullary nail is longitudinally placedwithin the medullary canal to contact at least two fragments, i.e., suchthat the nail extends on both sides of the fracture. As used herein, theterm “fragment” refers to a portion of a fractured bone regardless ofwhether the fracture is complete. When implanted, the nail strengthensand supports fragments of the fractured bone during healing of thefracture.

Various types of intramedullary nails are well known within the medicaldevice arts, and several different methods have been used to attach theintramedullary nail within the bone. For instance, in U.S. Pat. No.4,338,926 to Kummer et al., an intramedullary nail is disclosed whichplaces a compressive force radially outward on the interior wall of thecortex structure surrounding the intramedullary nail. The compressiveforce secures the Kummer nail within the medullary canal of thefragments. Similarly, in U.S. Pat. No. 4,457,301 to Walker a flexibleplastic core elements holds longitudinal pins of an intramedullary nailin place. In U.S. Pat. No. 5,514,137 to Coutts, cement is injectedthrough a cannula in an intramedullary nail to secure the distal end ofthe intramedullary nail to the bone. Other intramedullary nail designsemploy a more secure and mechanically positive attachment to the bone,such as through use of one or more bone fasteners which extendtransversely to the longitudinal axis of the nail and through the cortexof the bone. The bone fastener is received within a receiving recess orthrough-hole within the intramedullary nail to secure the intramedullarynail relative to the bone fastener. In the transverse attachment, thereceiving opening defines an axis which is at an angle to thelongitudinal axis of the nail (90° and 45° angles are common), and thebone fastener is advanced on this receiving opening axis. U.S. Pat. No.4,733,654 to Marino, U.S. Pat. No. 5,057,110 to Kranz et al., U.S. Pat.No. 5,127,913 to Thomas, Jr., U.S. Pat. No. 5,514,137 to Coutts(proximal end) and others disclose such a transverse bone fastenerattachment in a bicortical attachment. U.S. Pat. No. 5,484,438 to Pennigshows a nail design with a recess which permits only unicorticalattachment. The present invention particularly relates to intramedullarynails which use bone fasteners transversely through the cortex forattachment.

Problems may arise when attaching an intramedullary nail to a fragmentwith a bone fastener. It is occasionally difficult for the surgeon toproperly align the bone fastener and/or a hole for the bone fastenerwith the receiving opening on the nail. Part of the error is simply dueto difficulty in aligning the bone fastener with the receiving openingwhen the receiving opening is within the bone. Additionally, the nailmay be slightly bent during insertion of the nail structure into themedullary canal. Such bending of the nail structure may be desired insome instances so the nail shape better matches the particular shape ofthe medullary canal for a particular patient. Regardless of whetherintended or unintended, bending of the nail structure creates furtheralignment errors between the bone fastener and/or a hole for the bonefastener and the receiving opening on the nail. Four types of alignmenterrors can be identified: (a) in transverse displacement (e.g., when theaxis of the bone fastener is in the same transverse plane as thereceiving opening in the nail but does not intersect the axis of thenail), (b) in longitudinal displacement (i.e., when the bone fastener isat a different longitudinal location than the receiving opening in thenail), (c) in longitudinal angular misalignment (i.e., when the axis ofthe receiving opening and the axis of the bone fastener are at differentangles relative to the longitudinal axis of the nail), and (d) intransverse angular misalignment (i.e., when the axis of the receivingopening and the axis of the bone fastener are in the same transverseplane but at different radial positions relative to the nail).

Various types of jigs have been proposed to reduce alignment errors,such as shown in U.S. Pat. No. 4,733,654 to Marino and U.S. Pat. No.5,776,194 to Mikol et al. The jig may be temporarily attached to theproximal end of the nail to help align the bone fastener and/or thedrill to the receiving opening in the nail. While such jigs are helpful,they become less reliable as distance from the proximal end of the nailincreases, particularly if any bending of the intramedullary nail hasoccurred. Additional solutions are needed, especially for attaching thedistal end of the intramedullary nail to a distal fragment.

A second method to reduce such alignment problems is to locate thereceiving openings in-situ, such as through an x-ray or through the useof magnets as taught in U.S. Pat. No. 5,127,913 to Thomas, Jr. Suchmethods are not typically preferred by surgeons in as much as theyrequire significant additional time and effort during the orthopedicsurgery, to the detriment of the patient.

A third method to reduce such alignment problems is to drill thereceiving opening into the intramedullary nail only after the nail isplaced into the bone, allowing the receiving opening to be formed at arange of locations. Such insitu drilling is taught in U.S. Pat. No.5,057,110 to Kranz et al., wherein a tip section of the intramedullarynail is formed of a bioresorbable material. However, bioresorbablematerials are not as strong as metals, leading to a product which isweaker than desired and has a weaker attachment than desired.

Further problems with intramedullary nails occur during placement of theintramedullary nail. For minimal damage to cortical tissue of the boneand most beneficial healing, both the hole that is drilled in themedullary canal for the nail and then the nail itself need to beprecisely located and secured with respect to the medullary canal.

Additional problems with intramedullary nails occur due to the healingrequirements of the bone with respect to the strength and rigidity ofthe nail. U.S. Pat. No. 4,756,307 to Crowninshield and U.S. Pat. No.4,338,926 to Kummer et al. disclose intramedullary nails withbioresorbable portions to weaken the nail relative to the bone overtime, but these nails forsake the use of a transverse bone fastener toachieve this benefit.

BRIEF SUMMARY OF THE INVENTION

The present invention is an intramedullary nail for treatment of afracture of a bone by placement of the intramedullary nail within themedullary canal of the fractured bone. The nail structure is formed withat least one window in an exterior side, and a spacer of a non-metalmaterial is positioned within the window. In one aspect of theinvention, the non-metal spacer is formed of a bioresorbable material,and the window is a dynamization window. The nail is used with a bonefastener such as a bone screw which is advanced transversely through thebone and into the spacer, preferably in a bicortical attachment with thebone. The bone fastener is smaller across than the window, so the spacerspaces the bone fastener relative to the metal structure of the nail.The window may have a longitudinal length that is different from itswidth, while the bone fastener has a circular cross-section. Because thebone fastener is smaller across than the window and spacer, a largererror in placement of the bone fastener is permissible. Also, as thebioresorbable spacer resorbs, stress is increasingly transmitted throughthe fracture site rather than through the intramedullary nail. Thepositioning of the bone fastener, the shape and size of the window andspacer, and the material of the spacer all allow design control over thetype and amount of dynamization seen at the fracture site.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of an intramedullary nail in accordancewith the present invention.

FIG. 2 is a cross-sectional view taken along lines 2—2 in FIG. 1.

FIG. 3 is a cross-sectional view taken along lines 3—3 in FIG. 2.

FIG. 4 is a cross-sectional view taken along lines 4—4 in FIGS. 1 and 3.

FIG. 5 is a cross-sectional view taken along lines 5—5 in FIGS. 1 and 3.

FIG. 6 is a cross-sectional view taken along lines 6—6 in FIGS. 1 and 3.

FIG. 7 is a cross-sectional view of a distal end of the nail of FIGS.1-6 in a first type of attachment to a bone.

FIG. 8 is a cross-sectional view taken along lines 8—8 in FIG. 7.

FIG. 9 is a cross-sectional view of a distal end of the nail of FIGS.1-6 in a second type of attachment to a bone.

FIG. 10 is a cross-sectional view of a distal end of the nail of FIGS.1-6 in a third type of attachment to a bone.

FIG. 11 is a cross-sectional view of a distal end of the nail of FIGS.1-6 in a fourth type of attachment to a bone.

FIG. 12 is a cross-sectional view similar to FIG. 8 showing a first typeof permissible offset.

FIG. 13 is a cross-sectional view similar to FIG. 8 showing a secondtype of permissible offset.

While the above-identified drawing figures set forth one or morepreferred embodiments, other embodiments of the present invention arealso contemplated, some of which are noted in the discussion. In allcases, this disclosure presents the illustrated embodiments of thepresent invention by way of representation and not limitation. Numerousother minor modifications and embodiments can be devised by thoseskilled in the art which fall within the scope and spirit of theprinciples of this invention.

DETAILED DESCRIPTION

An intramedullary nail 20 according to the present invention includes anail structure 22 with a proximal end 24, a distal end 26 and a shaft28, with “proximal” and “distal” being defined in accordance with thedirection the nail 20 is intended to be inserted into the bone. As knownin the art, the dimensions of the proximal end 24, the distal end 26 andthe shaft 28 may be selected based on the required strength of the nailand the intended use of the intramedullary nail. The nail 20 depicted inFIGS. 1-13 is generally sized and shaped for treating a fracture towardthe middle of an otherwise healthy adult human femur. If desired, thenail 20 may be included in a kit having various sizes of nails to fitthe femurs of variously sized patients, and/or having various sizes ofnails to fit various types of femoral bone conditions or various typesof femoral fractures, and/or further having various sizes of nails tofit various other bones. For instance, the length of the femoral nail 20shown may be selected as needed between about 10 and 20 inches.

The distal end 26 may include a tip 30 having for instance a conical orpartially conical profile. The conical profile of the tip 30 aids ininserting the nail 20 into the medullary canal. The shaft 28 may begenerally of constant diameter. The proximal end 24 may include aportion of larger diameter than the shaft 28.

As shown in FIGS. 4-6 and known in the art, the nail 20 has an overallcross-sectional shape selected based upon the intended use. For afemoral nail 20, the cross-sectional shape may be generally circular, tomatch the generally circular cross-sectional shape of the medullarycanal of a healthy femur. For instance, the shaft 28 may be generallyformed with an outside diameter of 0.394 inches.

As best shown in FIGS. 1, 4 and 6, shallow longitudinal recesses 32 maybe formed into the outside surface of the shaft 28. These longitudinalrecesses 32 help to increase blood supply through the endosteum of thebone and to the fracture site during healing. Other cross-sectionalshapes can be alternatively used for particular purposes or to bettermatch the cross-sectional shape of the medullary canal of the particularbone being treated.

A cannula 34 preferably extends the length of the nail 20. The cannula34 facilitates insertion and alignment of the nail 20 within themedullary canal. The cannula 34 may be formed at each of the ends 24, 26by drilling along the longitudinal axis 36 of the nail 20. In the shaft28 of the nail 20, the cannula 34 may be formed by cutting into the nail20 from one of the sides. Alternatively, the cannula 34 may be formed bydrilling longitudinally the entire length of the nail 20, which wouldresult in a shaft 28 which encloses the cannula 34. Because the naillength is great compared with the nail width, it is generally easier tofabricate the cannula 34 by cutting laterally through the side of theshaft 28 than by drilling the length of the nail 20.

The cannula 34 receives a guide wire (not shown) during insertion of thenail 20 into the medullary canal. The guide wire has to be thick enoughto provide the requisite strength and rigidity for placement into thebone, and the cannula 34 must be large enough to receive the guide wireand permit longitudinal travel of the nail 20 along the guide wire.Conversely, because a larger cannula 34 detracts from the strength ofthe nail 20, the cannula 34 should be as small as required for travelover the guide wire. The preferred guide wire is circular incross-section, and as shown in FIGS. 4-6 the preferred cannula 34generally matches this circular cross-section. For instance, the cannula34 may be about 0.156 inches in diameter. With a shaft 28 of 0.394 inch(10 mm) diameter, this cannula 34 leaves a wall thickness for the shaft28 of about 0.118 inches.

The preferred nail 20 includes a large radius bend 38 in the shaft 28,generally intended to match the anterior-posterior bend of a healthyfemur. The bend 38 may have a large radius in relation to the length ofthe nail 20, such as a bend with a radius of 2 to 10 times the length ofthe nail 20. The curvature of the bend 38 may be applied over only acentral portion of the shaft 28, leaving the proximal end 24 and distalend 26 straight. For instance, the bend 38 may be applied over a central5 to 13 inches of the nail 20, depending on nail length.

Other than the cannula 34 being open from only one side of the shaft 28,the nail 20 is preferably symmetrical about a bisectinganterior-posterior plane. This allows the nail 20 to be used in eitherthe right or left femur while still maintaining the bend 38 appropriatefor the curvature of the femur.

The nail structure 22 is formed of a structurally strong biocompatiblematerial as known in the art. For instance, the nail structure 22 can beformed of a single piece of metal, with the preferred metal beingtitanium, such as a Ti-6AL-4V ELI titanium per ASTM F-136.

The proximal end 24 is preferably formed with one or more through-holes40 to facilitate attachment to a proximal bone fragment. For instance,the proximal end 24 may include two holes 40 which intersect each other.As best shown in FIG. 2, each of these holes 40 preferably extends at anangle α relative to the longitudinal axis 36 of the nail structure 22,with the preferred angle α being about 46°. Both the holes 40 preferablyextend at an anteversion angle β of about 15°, posteriorly (downward asshown in FIG. 2) on the proximal side and anteriorly (upward as shown inFIG. 2) on the distal side. These holes 40 allow attachment to a femoralfragment by bicortical attachment and with either antegrade fixation(i.e., through the trochanter) or reconstruction fixation (i.e., intothe femoral head) as selected by the orthopedic surgeon. Alternativelyor in conjunction with the through-holes 40, one or more recesses orcavities (not shown) may be provided in the proximal end 24 to permitunicortical attachment of the proximal end 24.

The proximal end 24 of the nail structure 22 may further includestructure to facilitate attachment of a drilling or aligning jig (notshown) as known in the art for placement of bone fasteners relative tothe nail 20. For instance, a proximal opening 42 aligned along thelongitudinal axis 36 may be used to receive an end of a jig in a matingrelationship. Workers skilled in the art will appreciate that numerousother structures could be equivalently used to temporarily hold the jigrelative to the nail 20.

The distal end 26 of the nail structure 22 includes at least onedynamization window 44 through an external surface, with a spacer 46 inthe dynamization window 44. The term “window” as used herein refers toan opening on an exterior surface of the nail 20. These windows 44 arereferred to as “dynamization” windows because, when used in conjunctionwith a properly dimensioned bone fastener (shown in FIGS. 7-13) and witha spacer 46 formed of a bioresorbable material, the proportion of stresscarried by the nail 20 relative to stress carried by the healing boneacross the fracture site dynamically changes as a function of time.

If desired, a single dynamization window or cavity may be provided,which would permit only unicortical attachment. In the preferredembodiment, two dynamization windows 44 are provided on opposite sidesof the nail 20, with each dynamization window 44 extending entirelythrough the side wall of the nail 20 to permit communication with thecannula 34. After removal of the guide wire from the cannula 34, thedynamization windows 44 permit bi-cortical attachment by inserting abone fastener through the cortex on one side of the nail 20, “in” onedynamization window 44, “out” the other dynamization window 44, andthrough the cortex on the other side of the nail 20. While the preferredembodiment includes only one set of dynamization windows 44, additionaldynamization windows may be located at other longitudinal locations ofthe nail 20, including the proximal end 24 and the shaft 28 as well asthe distal end 26. Any additional dynamization windows may either besingle dynamization windows permitting unicortical attachment oropposing sets permitting bicortical attachment. Any additionaldynamization windows may either be perpendicular to the bisecting planeor at other angles through the nail 20.

In the preferred embodiment, the dynamization windows 44 are aligned onopposite sides of the nail 20 at the same longitudinal location. Withthis configuration, both dynamization windows 44 may be simultaneouslyformed by a single cutting tool advanced through the nail 20 in adirection perpendicular to the bisecting plane. Alternatively, twodynamization windows may be longitudinally (and/or radially) offset withrespect to each other and still permit bicortical attachment, providedthey sufficiently overlap to permit the bone fastener to simultaneouslypass through both windows.

A spacer 46 is placed in each dynamization window 44. During use of thenail 20 as shown in FIGS. 7-13, a bone fastener 48 is positioned intothe dynamization window 44, and the spacer 46 spaces the bone fastener48 relative to the nail structure 22 defining the dynamization window44. Force is transmitted between the nail structure 22 and the bonefastener 48 primarily as a compressive load on a portion of the spacer46.

Each spacer 46 is formed of a non-metal material, and preferably of abioresorbable material. The term “bioresorbable” as used herein refersto any biocompatible material which dissolves or degrades over timeafter implantation into the human body. Among others, possiblebioresorbable materials include polymers and copolymers glycolic acid,lactic acid, aminocaproic acid, lactides, desoxazon, hydroxybutric acid,hydroxyvaleric acid, hydroxymethacrylate, peptides, polyesters ofsuccinic acid and cross-linked hyaluronic acid, or even a biologicallyabsorbable hydroxyapatite or tricalcium phosphate. The preferredbioresorbable material is a polylactic acid (“PLA”), which provides astrong material for the spacers 46. The compressibility of the PLAmaterial shows little change over the first few weeks of implantation,but then increases linearly over the next few months until resorption tothe point where the material will no longer support a load. With thepreferred PLA material, full resorption will typically occur withinabout two to five years.

The dynamization windows 44 and the spacers 46 are shaped based on therequired strength and the desired dynamization characteristics for thenail 20. In the preferred embodiment as shown in FIGS. 1 and 3, both thefirst and second dynamization windows 44 and spacers 46 have circularends 50 and a rectangular central section 52, and the spacers 46 fillthe dynamization windows 44 in length and width. As will be furtherexplained with reference to FIGS. 7-13, this shape provides substantiallongitudinal dynamization flexibility while still providing adequatestrength for the nail 20 at the dynamization windows 44. In the 0.394inch (10 mm) OD nail 20 and for use with 0.177 inch (4.5 mm) OD bonescrews 48, each end 50 may have a circular radius of 0.124 inches, withthe central section 52 being 0.345 inches in length and 0.248 inches inwidth, for a total dynamization window length of 0.611 inches.Alternatively each spacer may not completely fill its dynamizationwindow, such as by not being either full width or full length (whichcontrols whether force transmitted through the spacer is in compression,in tension or in shear), or by having a central opening through eachspacer.

As best shown in cross-sectional views of FIGS. 2 and 5, each spacer 46has an exposed surface which preferably has a shallow groove 54 in thecenter. The groove 54 may provide a “V” shape to the exposed surface ofthe central section 52 of the spacer 46, while the exposed surface ofthe ends 50 of the spacer 46 may be conical. In the preferred embodimentshown, the groove 54 is about 0.04 inches (1 mm) deeper than the edgesof the spacer 46. During surgical implantation of the bone fastener 48into the nail 20, the groove 54 assists in directing the guidepin/drill/bone fastener toward the center of the spacer 46. Workersskilled in the art will appreciate that numerous alternative surfacecontours may be selected for one or both spacers 46 which still providea generally sloped surface directing the guide pin/drill/bone fastenerinward toward a center of each spacer 46.

In the preferred embodiment as best shown in FIG. 5, the two opposingspacers 46 are formed as a single insert 56. For instance, with a 0.394inch (10 mm) OD nail 20, the insert 56 may have an overall thickness ofabout 0.286 inches (7.3 mm). Alternatively, each spacer 46 may beseparately formed.

A cannula 58 is formed in the insert 56 to correspond with the cannula34 of the nail structure 22, such that the two spacers 46 are defined onopposing sides of the cannula 58. With the center of the groove 54 onthe outside and the cannula 58 toward the inside, the center of eachspacer 46 may be quite thin. For instance, with a cannula 58 of about0.156 inches (3.9 mm) in diameter, the center of each spacer 46 may beonly about 0.025 inches (0.6 mm) thick.

As an alternative to the groove 54, the spacer 46 may include an exposedsurface which is planar. Depending upon the material of the spacer 46and the thickness of the spacer 46 relative to the cannula 58, thecenter of the spacer 46 may be resiliently deflected or deformed inwardunder pressure. For instance, the push force placed on the spacer 46 bythe guide pin, drill and/or bone fastener during insertion through thespacer 46 may cause the center of the spacer 46 to resiliently deform,such that an exposed surface which was planar as manufactured provides asloping profile which assists in directing the guide pin/drill/bonefastener toward the center of the spacer 46.

The preferred bioresorbable material is commercially available such asin about 150 in³ blocks. The insert 56 may be formed by cutting thebioresorbable material to ⅝ inch by ⅝ inch by 3 inch portions, which maythen be further fabricated to the shape of the insert 56 by CNC. Thecannula 58 is preferably drilled into the insert 56 prior to insertionof the insert 56 into the nail 20, although the cannula 58 mayalternatively be drilled after placing the insert 56 into the nail 20,either simultaneously with or after formation of the cannula 34 throughthe nail structure 22.

The insert 56 preferably fits into the dynamization windows 44 with apress fit. The insert 56 is pressed in the nail 20 until it alignscentrally within the nail 20. Initial results have indicated thatseveral hundreds of pounds of press force is required to press thepreferred insert 56 into the windows 44 of the preferred nail structure22. During the surgery, the insert 56 can be drilled through with a pushforce which is at least an order of magnitude less than the press force,and the press fit amply secures the insert 56 into the nail 20.

Workers skilled in the art will appreciate that there are many otherways to attach the spacers 46 into the dynamization windows 44. Variousrecesses or protrusions on the spacers 46 and/or in the nail structure22 may provide a higher pull strength or facilitate a positively securedattachment of the spacers 46 to the nail structure 22.

Attachment of the spacers 46 into the dynamization windows 44 does nothave to be performed as a manufacturing step. Alternatively, the surgeonmay attach the spacers 46 into the dynamization windows 44 as apreparatory step during surgery, and the nail structure 22 and spacers46 may be appropriately modified to facilitate placement of the spacers46 into the dynamization windows 44 by the surgeon. For instance, theinsert 56 and dynamization windows 44 may have a smaller amount ofinterference to enable the surgeon the press the insert 56 into the nailstructure 22 by hand. Alternatively, the surgeon may be provided with amechanical press to facilitate pressing the insert 56 into the nailstructure 22. If desired, a lubricant may be utilized to facilitate thepress fit. The lubricant used may be volatile, so the insert 56 becomestightly secured into the nail structure 22 after the lubricantevaporates. As another alternative, the insert 56 and the dynamizationwindows may be sized with a slight clearance and be adhesively secured.Any lubricant or adhesive should be biocompatible so as to not createcomplications in the healing process.

Attachment of the insert 56 into the dynamization windows 44 by thesurgeon allows several further advantages. For instance, a single nailstructure 22 may be provided as part of a kit which includes a pluralityof inserts 56 having different properties. The different inserts 56provided may have different mechanical properties, such as differenthardnesses, different rates of absorption, etc., allowing the surgeonthe flexibility to match the insert 56 used with the particular healingmodality desired by the surgeon.

The non-metallic spacers 46 may also include one or more active agentsto promote effective healing. For instance, the non-metal material ofthe spacers 46 may include one or more antibiotics such as gentamicin,methicillin, penicillin, etc. The non-metal material of the insert 56may also include other active agents, such a one or more of atransforming growth factor—beta 1, a recombinant human bonemorphogenetic protein—2, etc. If provided as part of a kit, differentinserts 56 may be provided each with a different active agent or adifferent amount of active agent, so the surgeon can select the type andamount of active agent used for the particular surgery.

Another advantage of attachment of the insert 56 into the dynamizationwindows by the surgeon is that the insert 56 may be handled in adifferent environment from the nail structure 22. For instance, theinsert 56 may be maintained in a particular thermal condition (e.g.,refrigerated or frozen), or in a sealed container (e.g., sealed fromair, sealed from humidity, etc.) until immediately prior to insertioninto the dynamization windows 44 and immediately prior to implantationinto the fractured bone. The controlled environment of the insert 56 mayhave beneficial results in physical properties (e.g., preventingdissipation or dilution of an active agent, etc.) or in mechanicalproperties (e.g., increased hardness, different size due to thermalexpansion, etc.) of the insert 56 upon implantation.

The distal end 26 of the preferred nail 20 preferably includes anon-dynamic through-hole 60. The through-hole 60 has an axis 62 which ispreferably perpendicular to the anterior-posterior plane andintersecting the longitudinal axis 36 of the nail 20. The through-hole60 defines a first window 64 into the cannula 34 and a second window 64out of the cannula 34 at the opposite side of the nail 20. Each window64 may be circular in cross-section, and both windows 64 may be definedwith a single drilling operation. The size and shape of the windows 64are selected based on the intended bone fasteners to be used. Forinstance, both windows 64 may be circular with a 0.217 inch diameter.For bi-cortical attachment of the distal end 26 of the nail structure 22using the through-hole 60, a bone fastener 48 is advanced through thethrough-hole 60, i.e., through both windows 64. While the preferredembodiment includes only one set of non-dynamization windows 64,additional non-dynamization windows 64 may be located at otherlongitudinal locations of the nail 20, including the proximal end 24 andthe shaft 28 as well as the distal end 26. Any additionalnon-dynamization windows may either be single windows permittingunicortical attachment or opposing sets permitting bicorticalattachment. Any additional non-dynamization windows may either beperpendicular to the bisecting plane or at other angles through the nail20.

The bone fasteners 48 used with the nail 20 may be for instance bonepins or bone screws, sized and shaped as appropriate for the site ofimplantation. Each bone fastener 48 may be directly implanted into thecortex, or a hole may be drilled or otherwise opened in the cortex priorto placement of the bone fastener 48. The bone pin or bone screw may besolid, or may be cannulated such as for implantation over a guide pin.In the preferred embodiment, the distal through-hole 60 is sized toreceive 0.177 inch (4.5 mm) outside diameter bone screws, and thedynamization windows 44 and spacers 46 are sized appropriately for 0.177inch (4.5 mm) outside diameter bone screws. The proximal through-holes40 as preferably sized appropriately for 0.256 inch (6.5 mm) outsidediameter bone screws. Other types of bone fasteners may be alternativelyused at the option of the orthopedic surgeon.

FIGS. 7-13 show various attachment configurations for the nail 20 of thepresent invention. FIGS. 7 and 8 show a bicortical attachment with asingle bone screw 48 positioned at the distal end of the twodynamization windows 44, which can be characterized as an “initialdynamic” locking position. Attached in this position, the nail 20provides only compressive dynamization across the fracture site 66, asfollows. The bioresorbable spacer 46 can be thought of as a compressionspring with a time-varying spring constant, positioned within asubstantially incompressible nail structure 22. In the attachment shownin FIGS. 7 and 8, substantially the entire length of the “spring” is onthe proximal side of the bone fastener 48. Very little force istransmitted through the nail 20 until the bone is loaded. When thefractured bone is loaded in compression, the compressive load is carriedacross the fracture site 66 by the nail shaft 28 and then through theproximal length of the spacer 46, and then to the bone fastener 48 anddistal fragment 68. Initially on implantation, the bioresorbable spacer46 is very rigid and hard, and substantially incompressible like thenail structure 22. The nail 20 will carry substantially all of thecompressive force, and none of the compressive force will be carriedacross the fracture site 66.

After the bone begins healing, such as after several weeks, thebioresorbable material begins to deteriorate. This increases thecompressibility (lowers the spring constant) of the bioresorbablematerial in the dynamization window 44. In this condition, when acompressive stress is placed across the fracture site 66, the proximalside of the spacer 46 will compress slightly under the load. Because ofthis slight compression, significant amounts of the compressive stresswill be carried by the healing bone as well as by the nail 20.

As the bioresorable material further deteriorates, the proportion ofstress carried by the nail 20 relative to stress carried by the healingbone continues to decrease. The healing bone continues to be dynamized,until substantially all compressive stresses placed on the bone arecarried across the fracture site 66 rather than by the nail 20.

Most of the stresses carried by the bone are compressive stresses ratherthan tensile stresses. Nonetheless, in contrast to the compressivedynamization, consider the path of tensile stress placed on the bonewhen the nail 20 is attached as shown in FIGS. 7 and 8. When the bone isloaded in tension, the tensile stress is carried across the fracturesite 66 by the nail shaft 28 and then around to the distal side of thedynamization window 44 by the nail structure 22, then transferred as acompressive stress through only a small distal length of the spacer 46,and then to the bone fastener 48 and distal fragment 68. Because thebone fastener 48 is quite close to the distal end of the dynamizationwindow 44, there is a very short length of bioresorbable material toundergo compression, and there is very little give in the short distallength of bioresorbable material regardless of the amount ofdeterioration. Tensile stresses placed across the fracture site 66 arealmost entirely borne by the nail 20, regardless of deterioration of thebioresorbable spacer 46.

FIG. 9 shows an alternative attachment of the nail 20, which can beeither a “static” locking position or a “delayed dynamic” lockingposition depending upon screw removal. In this static locking position,the nail 20 is attached with a first bone screw 48 through the openthrough-hole 60 and a second bone screw 48 through a distal end of thedynamization windows 44. The two screw attachment helps further securethe distal fragment 68 to the nail 20, and particularly helps to preventany rotational movement or “toggling” of the distal fragment 68 whichmight otherwise occur about a single screw. Toggling of the distalfragment 68 may particularly be a problem if the distal end 26 of thenail 20 does not fit securely and tightly within the medullary canal ofthe distal fragment 68.

With two screw attachments and particularly with the screw 48 throughthe open through-hole 60, there is very little dynamization which isinitially seen by the fracture. However, an intermittent operation maybe performed after initial healing of the fracture in which the bonescrew 48 through the open though-hole 60 is removed, resulting in thedelayed dynamic configuration. With a single screw attachment throughthe distal end of the dynamization windows 44, compressive dynamizationof the fracture will be achieved after the intermittent operation.

If a completely static attachment is desired, the recommendedpositioning of bone screws 48 includes a first screw 48 through the openthrough-hole 60 and a second bone screw 48 through a proximal end of thedynamization windows 44 as shown in FIG. 10. This positioning allowsmaximum separation between the bone screws 48 for toggle prevention andmaximum strength. For each of the initial dynamic, the delayed dynamicand the completely static attachments, the surgeon can further adjustbone screw positioning as necessary for the condition of the bone.

In an alternative nail design (not shown) having two distal sets ofdynamization windows 44, toggling of the distal fragment 68 will beprevented by a two screw attachment while full dynamization can beachieved without removal of either screw.

Many middle grounds or intermediate longitudinal locations can also beselected by the surgeon for placement of the bone screw 48 through thedynamization windows 44. By selecting the longitudinal location of thebone screw 48 through the dynamization windows 44, the surgeon canselect the proportion of compressive dynamization and tensiledynamization seen at the fracture site 66.

The dynamization windows 44 are significantly longer than the width ofthe intended bone fastener 48. Because of this, while the exactlongitudinal location of the bone fastener 48 is important for thedesired dynamization, the exact longitudinal location is not critical touse of the nail 20. Minor longitudinal displacement errors of the bonefastener 48 will not prevent the bone fastener 48 from being advancedthrough the nail 20. The preferred nail structure 22 permitslongitudinal displacement of the preferred bone fastener 48 up to amaximum of 0.434 inches while still receiving the bone fastener 48through both windows 44. This large range of longitudinal location ofthe bone fastener 48 relative to the dynamization windows 44 not onlyprovides permissible error for the surgeon, but also allows the surgeonflexibility in placement of the bone fasteners 48 relative to thefracture and relative to changes in bone condition at differentlongitudinal locations.

FIGS. 11-13 further show how the present invention provides flexibilityin locating the bone fastener(s) 48 relative to the intramedullary nail20 and also in providing for a range of error in locating the bonefastener(s) 48 relative to the nail 20. These benefits are achieved dueto the different mechanical properties (such as hardness) of thenon-metal material of the spacers 46, regardless of whether thenon-metal material chosen is bioresorbable.

The longitudinal length of the two windows 44 with respect to each otherallows for a significant longitudinal angulation γ of the bone screw 48relative to the nail 20, such as up to about 45° as shown in FIG. 11.Three factors may result in the longitudinal angulation γ of the bonescrew 48. Firstly, the location of the bone fastener 48 shown in FIG. 11may result in a bending dynamization of the fracture site 66. The bonefastener 48 contacts the nail 20 at a proximal end 70 of onedynamization window 44 and at a distal end 72 of the other dynamizationwindow 44. Tensile loads are transmitted through the distal end 72contact without dynamization, and compressive loads are transmittedthrough the proximal end 70 contact without dynamization. However,bending stress such as that created by placing a clockwise (in FIG. 11)moment on the distal fragment 68 may allow dynamization. The extent ofbending dynamization of the fracture site 66 depends on how secure thedistal end 26 is in the medullary canal of the distal fragment 68. Aloose fit of the distal end 26 in the distal fragment 68 will allow somerotational play, and the compressibility of the spacer material willgovern how much bending stress is transferred through the fracture.Conversely, a tight fit of the distal end 26 in the distal fragment 68will prevent any clockwise bending dynamization, as the distal fragment68 cannot rotate relative to the nail 20 due to the tight fit. A loosefit of the distal end 26 in the distal fragment 68 may result eitherfrom the condition of the original bone or due to widening the medullarycanal during surgery relative to the diameter of the nail 20. If thesurgeon wishes clockwise bending dynamization to occur, first a loosefit must be obtained, and then the bone fastener 48 is placed throughthe dynamization windows 44 as shown in FIG. 11. Through properlongitudinal angulation γ of the bone fastener 48, the structure of thepreferred nail 20 thus allows the surgeon to select whether, how much,and in which direction bending dynamization occurs.

A second reason for longitudinal angulation γ of the bone fastener 48 isbased on the condition of the fracture. With longitudinal angulation γof the bone fastener 48, the bone fastener 48 extends through one sideof the cortex at a position longitudinally offset from the location thebone fastener 48 extends through the other side of the cortex. Thesurgeon may determine that significant longitudinal angulation γ isnecessary for best securement of the bone fastener 48 relative to thefracture location(s).

A third reason for longitudinal angulation γ of the bone fastener 48 ismerely due to longitudinal angular misalignment of the bone fastener 48.The axis of the bone fastener 48 may be angularly misaligned relative toits desired position. The structure of the preferred nail 20 permitslongitudinal angular misalignment of the bone fastener 48 while stillreceiving the bone fastener 48 through both windows 44.

As best shown in FIG. 12, the width of the two windows 44 is preferablygreater than the width of the bone fastener 48. This difference in widthpermits some transverse displacement 74 of the bone screw 48 withrespect to the longitudinal axis 36 of the nail 20, either by error oras intended by the surgeon. The structure of the preferred nail 20 inconjunction with the preferred bone fastener 48 permits a transversedisplacement 74 up to a maximum of 0.071 inches. Because the spacermaterial is drilled in-situ or the bone fastener 48 used opens its ownhole through the spacer 46, the spacer 46 holds the bone fastener 48securely with respect to the nail 20 anywhere within the dynamizationwindows 44, at least until resorption of the spacer 46 becomessignificant.

As best shown in FIG. 13, because the width of the windows 44 is greaterthan the width of the bone screw 48, some amount of transverseangulation δ is also permitted. Similar to transverse displacement 74,this transverse angulation δ may either be the result of error or beintended by the surgeon. The structure of the preferred nail 20 permitsa transverse angulation δ with the preferred bone fastener 48 up to amaximum of about 11° from the axis of the dynamization windows 44.

The preferred PLA material for the spacers 46 and the preferred shape ofthe spacers 46 provide very useful general purpose dynamizationcharacteristics based on currently known information about how bonefractures heal. The present invention further introduces an entirely newscience to bone healing. That is, as explained with regard to thepreferred embodiment, the selection of the bioresorbable materialdetermines its compressibility curve as a function of resorption time.Different bioresorbable materials have different compressibility curves,affecting the dynamization seen at the fracture site 66. Differentspacer geometries and different bone fastener locations and geometriesalso affect the dynamization (tensile, compressive and bending) seen atthe fracture site 66. The present invention will allow a new body ofdata to be gathered on the effectiveness of bone fracture healing underdifferent dynamization conditions. Based on this data, future changesmay be made to further improve the invention, or to modify the inventionfor particular bone or fracture conditions. For instance, not only may adifferent bioresorbable material be used to change the compressibilitycurve, but a combination of bioresorbable materials may be used.Composite bioresorbable materials may be formed to combinecompressibility characteristics, or the spacer(s) 46 may be formed oftwo or more distinct bioresorbable materials. The thickness of these twoor more materials may be selected to engineer the desiredcompressibility curve of the spacer 46 and thereby provide the mostbeneficial dynamization characteristics. The bone fastener 48 may bepositioned in the dynamization window 44 between a proximal spacerportion of one material and a distal spacer portion of a second materialso as to have tensile dynamization characteristics which differ fromcompressive dynamization characteristics. The spacers 46 in opposingwindows 44 may be of different sizes or formed of differentbioresorbable materials to control the bending dynamization relative tothe tensile and compressive dynamization. The present invention thusallows controlled dynamization across the fracture site 66, both forimproving fracture healing and for learning more about how dynamizationaffects the healing of the fracture.

The preferred PLA material does not include any active agents forrelease during bioresorption. Alternatively, the bioresorbable materialmay include an active agent as desired for release adjacent the fracturesite, such as an antibiotic or a growth factor.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. For instance, while all of the theseattachments methods have been described with regard to the preferredbicortical attachment, unicortical attachment can also be used with ashorter bone fastener or by only advancing the bone fastener partiallythrough the nail 20.

What is claimed is:
 1. An intramedullary nail for treatment of afracture of a bone having a medullary canal extending longitudinally,comprising: a nail structure extending longitudinally and formed ofmetal, the nail structure having a distal end with a tip for insertioninto the medullary canal and a proximal end opposite the distal end,with a first window defined in an exterior side of the distal end of thenail structure, the first window having a first window longitudinallength and a first window width not equal to the first windowlongitudinal length; wherein the nail structure includes an openinglongitudinally spaced from the first window and in the distal end of thenail structure; and a first spacer formed of a non-metal material withinthe first window.
 2. The intramedullary nail of claim 1, wherein thenail structure further comprises a nail structure cannula definedlongitudinally therein, wherein the nail structure further comprises asecond window opposing the first window, with a second spacer in thesecond window, wherein the first spacer and the second spacer are formedby a single insert with an insert cannula defined therethrough andaligned with the nail structure cannula.
 3. The intramedullary nail ofclaim 1, wherein the nail structure includes a bend such that alongitudinal axis of the nail structure lies within a bisecting plane,wherein the nail structure further comprises a second window opposingthe first window permitting bicortical attachment with a bone fastenerthrough the first and second windows, and wherein the first window andthe second window are symmetrically disposed on opposing sides of thebisecting plane.
 4. The intramedullary nail of claim 1, wherein anadditional bone attachment hole is defined in a proximal end of the nailstructure.
 5. The intramedullary nail of claim 1, wherein the non-metalmaterial of the first spacer is a bioresorbable material; wherein thefirst window longitudinal length is greater than the first window width;wherein the nail structure further comprises: a second window defined inan exterior side of the nail structure opposing the first windowpermitting bicortical attachment with the bone fastener through thefirst and second windows, the second window having a second windowlongitudinal length and a second window width less than the secondwindow longitudinal length; a bend such that a longitudinal axis of thenail structure lies within a bisecting plane, wherein the first windowand the second window are symmetrically disposed on opposing sides ofthe bisecting plane; a nail structure cannula defined longitudinally inthe nail structure; and an additional bone attachment hole defined in aproximal end of the nail structure; wherein the opening longitudinallyspaced from the first window and in the distal end of the nail structureis a through-hole; and wherein a bioresorbable insert provides the firstspacer, the bioresorbable insert further providing a second spacer, thefirst spacer filling the first window and the second spacer filling thesecond window prior to anchoring a bone fastener through the first andsecond windows; and wherein an insert cannula is defined through thebioresorbable insert and aligned with the nail structure cannula.
 6. Theintramedullary nail of claim 1, wherein the first spacer is formed of anon-metal material separately from the nail structure, the first spacerhaving outer dimensions which correspond to the first window shape, suchthat the first spacer is insertable into the first window and receivedby the first window to secure the first spacer relative to the nailstructure.
 7. A bone support assembly for treatment of a fracture of abone, comprising: a bone support implant formed of metal, the bonesupport implant having a first window extending therethrough, the firstwindow having a first window shape, the bone support implant having alongitudinal direction for transferring stress in the longitudinaldirection upon implantation; and a first insert formed of a non-metalmaterial separately from the bone support implant, the first inserthaving outer dimensions which correspond to the first window shape, suchthat the first insert is insertable into the first window and receivedby the first window to secure the first insert relative to the bonesupport implant, the first insert having a bone support implantcontacting surface which contacts the bone support implant uponinsertion of the first insert into the first window, the bone supportimplant contacting surface extending generally normal to thelongitudinal direction of the bone support implant.
 8. The bone supportassembly of claim 7, wherein the bone support implant is anintramedullary nail for treatment of a fracture of a bone having amedullary canal extending longitudinally, and wherein the first windowis disposed in an exterior side of the distal end of the nail.
 9. Thebone support assembly of claim 7, wherein the bone support implantfurther comprises: a second window defined in the bone support implantopposing the first window permitting attachment with a bone fastenerthrough the first and second windows.
 10. The bone support assembly ofclaim 7, wherein the first window has a first window longitudinal lengthand a first window width, and wherein the first window longitudinallength is greater than the first window width.
 11. The bone supportassembly of claim 7, wherein the bone support implant further comprisesa cannula defined longitudinally therein.
 12. The bone support assemblyof claim 7, further comprising: a bone fastener having a lengthsufficient to extend through the first insert in attachment with a bone,the bone fastener having a width small enough to be received in theinsert and through the first window.
 13. The bone support assembly ofclaim 12, wherein the bone fastener is insertable through the insert inthe same direction as the insert is insertable into the bone supportimplant.
 14. The bone support assembly of claim 7, wherein the outerdimensions of the first insert are sized to be received in the firstwindow with a press fit.
 15. The bone support assembly of claim 7,wherein the first insert fills the first window prior to anchoring of abone fastener transversely through the first insert.
 16. A bone supportassembly for treatment of a fracture of a bone, comprising: a bonesupport implant formed of metal, the bone support implant having a firstwindow extending therethrough, the first window having a first windowshape, the first window having a first window longitudinal length; and afirst insert formed of a non-metal material, the first insert havingouter dimensions which correspond to the first window shape, such thatthe first insert is received by the first window to secure the firstinsert relative to the bone support implant; wherein the bone supportimplant has a proximal end and a distal end, wherein the first window isdisposed in one of the distal end and the proximal end of the bonesupport implant, wherein the bone support implant has an opening definedin said one of the distal end and the proximal end, the opening beinglongitudinally spaced from the first window such that the first windowand the opening are placed on the same side of the fracture, the openinghaving a longitudinal length which is less than the first windowlongitudinal length such that anchoring of a bone fastener transverselythrough the bone and into the opening can prevent dynamization whileanchoring of a bone fastener transversely through the bone and into thefirst window can permit dynamization.
 17. The bone support assembly ofclaim 16, wherein the first insert fills the first window prior toanchoring of a bone fastener.
 18. The bone support assembly of claim 16,wherein the non-metal material of the first insert is a bioresorbablematerial.
 19. The bone support assembly of claim 16, wherein the firstinsert is formed separately from the bone support implant, such that thefirst insert is insertable into the first window.
 20. A bone supportassembly for treatment of a fracture of a bone, comprising: a bonesupport implant formed of metal, the bone support implant having a firstwindow extending therethrough, the first window having a first windowshape; and a first insert formed of a non-metal material separately fromthe bone support implant, the first insert having outer dimensions whichcorrespond to the first window shape, such that the first insert isinsertable into the first window and received by the first window tosecure the first insert relative to the bone support implant; a secondinsert formed of a non-metal material separately from the bone supportimplant and separately from the first insert, the second insert havingouter dimensions which correspond to the first window shape, such thatthe second insert is insertable into the first window and received bythe first window to secure the second insert relative to the bonesupport implant, the second insert having different mechanical orchemical treatment properties than the first insert.
 21. The bonesupport assembly of claim 20, wherein the different mechanical orchemical treatment properties are selected from the group consisting of:different hardness, different rates of absorption, different activeagents and different amounts of active agents.
 22. A bone supportassembly for treatment of a fracture of a bone, comprising: a bonesupport implant formed of metal, the bone support implant having aproximal end and a distal end, the bone support implant having a firstwindow extending therethrough and positioned closer toward one of thedistal end and the proximal end, the first window having a first windowshape with a first window longitudinal length, the bone support implanthaving an opening defined in the bone support implant and positionedadjacent the first window in said one of the distal end and the proximalend, the opening being longitudinally spaced from the first window suchthat the first window and the opening are placed on the same side of thefracture, the opening having a longitudinal length which is less thanthe first window longitudinal length; and a first spacer formed of anon-metal material receivable in the first window, such that anchoringof a bone fastener transversely through the bone and into the openingcan prevent dynamization while anchoring of a bone fastener transverselythrough the bone and into the first window can permit dynamization. 23.The bone support assembly of claim 22, wherein the non-metal material ofthe first spacer is a bioresorbable material.
 24. The intramedullarynail assembly of claim 22, wherein the different mechanical or chemicaltreatment properties are selected from the group consisting of:different hardness, different rates of absorption, different activeagents and different amounts of active agents.
 25. A bone supportassembly for treatment of a fracture of a bone, comprising: a bonesupport implant formed of metal, the bone support implant having atleast one window defined therein for exposure of a selected spacer; afirst spacer formed of a non-metal material, the first spacer beingsized such that it is receivable in the window in an exposed positionfor transverse fastening through the bone support implant and throughthe first spacer with a bone fastener; and a second spacer formedseparately from the first spacer, the second spacer being sized suchthat it is receivable in the window in an exposed position fortransverse fastening through the bone support implant and through thesecond spacer with a bone fastener, the second spacer having differentmechanical or chemical treatment properties than the first spacer.